I think you are in Zone 6, in Harrisburg. This is a hardiness zone from a map based on usual minimum temperatures. It is important because you can grow plants that someone in the mountains north of you (zone 5) cannot grow.
When people say "a rock garden" they usually think of low growing plants, like creeping phlox. You have found out what is the limitation of many perennial (come back every year) flowers, many bloom only a short time. Annuals do not live through winter, like petunias, and you have to plant them every year. However most annuals don't stop blooming until the cold weather comes. I came up with a few ideas of colorful perennial flowers that I think would do well there, and bloom for much of the summer.
Dianthus- three varieties for you (there are lots and some are tall and not easy) try 'Fire Witch' and 'Zing Rose'.
Knautia 'Thunder and Lightening'
Aster 'Purple Dome' (many asters are too tall for a rock garden.
Daylily 'Stella D'Oro
Verbena 'Homestead Purple'
Veronica 'Giles van Hees'
The above are all "perennial" which means once you plant them, they come back every year, but the first year you might not get a lot of blooms. For a lot of color next year, go to Lowe's or any garden center, and buy a bunch of Portulaca starts, they are cheap and super colorful.
There is a great mail-order nursery in Ohio, called Bluestone Perennials. They have a nice website, probably carry all these perennials, and hundreds more. They have a plant-finding program you might enjoy now the weather is rotten. They also sell some 'pre-planned' garden collections you might look at.